qt5webengine: fix build issue with alsa

qt5webengine: fix build issue with alsa

Qt WebEngine auto-guesses if it compiles support for alsa. When the
alsa-lib config is enabled but the features mixer, rawmidi, hwdep and
seq are not also enabled, it leads to a build failure.

Lets the developer decide whether or not support for alsa in Qt
WebEngine by adding the new config BR2_PACKAGE_QT5WEBENGINE_ALSA that
selects the features it needs when it is set.

Fixes [1].

[1]: http://autobuild.buildroot.net/results/d81537a8f67bb0a3625057560b2f16daf4828f98/build-end.log

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>